fix(admin) fix data format for empty upstreams array
fix(admin) fix data format for empty upstreams array

When no active upstreams are present, the 'data' response field
should be an empty list, not an object

shiprabehera/kong,icyxp/kong,akh00/kong,Kong/kong,Mashape/kong,Kong/kong,Kong/kong,jebenexer/kong,salazar/kong